Differences in photosynthetic behaviour and leaf senescence of soybean (Glycine max [L.] Merrill) dependent on N2 fixation or nitrate supply

Abstract

Biological N(2) fixation can fulfil the N demand of legumes but may cost as much as 14% of current photosynthate. This photosynthate (C) sink strength would result in loss of productivity if rates of photosynthesis did not increase to compensate for the costs. We measured rates of leaf photosynthesis, concentrations of N, ureides and protein in leaves of two soybean cultivars (Glycine max [L.] Merrill) differing in potential shoot biomass production, either associated with Bradyrhizobium japonicum strains, or amended with nitrate. Our results show that the C costs of biological N(2) fixation can be compensated by increased photosynthesis. Nodulated plants shifted N metabolism towards ureide accumulation at the start of the reproductive stage, at which time leaf N concentration of nodulated plants was greater than that of N-fertilized plants. The C sink strength of N(2) fixation increased photosynthetic N use efficiency at the beginning of plant development. At later stages, although average protein concentrations were similar between the groups of plants, maximum leaf protein of nodulated plants occurred a few days later than in N-fertilized plants. The chlorophyll content of nodulated plants remained high until the pod-filling stage, whereas the chlorophyll content of N-fertilized plants started to decrease as early as the flowering stage. These results suggest that, due to higher C sink strength and efficient N(2) fixation, nodulated plants achieve higher rates of photosynthesis and have delayed leaf senescence.

Leaf Photosynthetic Metabolism and N2 Fixation at the Flowering Stage in Three Genotypes of Cowpea [Vigna unguiculata (L.) Walp.]

“…One of the first symptoms of nitrogen deficiency in the leaves is chlorophyll degradation, which causes a reduction in photosynthetic activity and TP content, characterizing the onset of leaf senescence. This early flowering response (35 DAE) of the Gurguéia cultivar supports the development of nodulated plants (Kaschuk et al, 2010), delaying leaf senescence, while maintaining high photosynthetic activity for longer (Lippi et al, 1999), thus preventing photosynthate deficiency in the nodules during the pod-filling stage, which consumes a large amount of nitrogen in the leaves.…”

“…Thus, the plants cannot sustain the high activity of the nodules and ensure high grain yields per plant, because their senescence begins quickly (Lippi et al, 1999). The situation is different for soybeans, which maintain high photosynthetic activity for a longer period in the early reproductive stage (Kaschuk et al, 2010), maintaining a high rate of sugar transport, protein synthesis and leaf chlorophyll content. This feature, a short period of peak photosynthesis in the preflowering phase, a common physiological behavior of bean, is perhaps the main reason that the culture responds to a small amount of mineral nitrogen while under BNF Hungary et al, 2003;Xavier et al, 2008;Pelegrin et al, 2009).…”

“…Another determining factor for the low efficiency of BNF in bean is the occurrence of water stress (Figueiredo et al, 2007;Hamidou et al, 2007), even in Vigna, which is more tolerant of water shortage than Phaseolus (Pimentel and Hébert, 1999). This is especially true if these stressful conditions occur between vegetative and early reproductive stages, when the production of new nodules is stabilizing in cowpea (Senanayake et al, 1987), leading to low efficiency of BNF during the reproductive stage when a high amount of nitrogen is required by the leaves in legumes (Neves et al, 1982;Kaschuk et al, 2010).…”

“…2B). Maintaining a high photosynthetic rate is critical to the efficiency of BNF (Kaschuk et al, 2010). The onset of leaf senescence in cowpea plants varies among cultivars (Lippi et al, 1999), leading the anticipation of the photosynthetic peak in cultivars with early senescence.…”

“…These changes may be the cause of low grain production in arid regions, such as northeastern Brazil, when under BNF conditions. In contrast, due to intensive research, excellent soybean production in Brazilian territory has occurred independent of mineral N fertilization (Kaschuk et al, 2010) and the success of BNF in this legume must be attributed to strong investment in technology and specific physiological characteristics.…”